Variation of carrier lifetime with optical excitation power density in micro-LEDs

Abstract

GaN-based micro-LEDs are applied to visible light communication due to their high modulation bandwidth with reduced chip size. It requires a deep understanding of recombination processes and their impact on the bandwidth, which is mainly determined by the carrier lifetime. We employed confocal time-resolved photoluminescence (TRPL) to characterize the variation of carrier lifetime with optical excitation power density on micro-LEDs. We observed an initial increase followed by a sudden decrease within the power density range of 96.7 kW/cm2 to 546 kW/cm2 on a blue micro-LED with a chip size of 80 µm. We attribute this phenomenon to increased optical excitation power, gradually saturating the defect-dominated non-radiative recombination centers, with radiative recombination processes gradually taking over. We compared the power density at the inflection point for different regions on the sample and the samples with different sizes and sidewall structures. The power density for the lifetime inflection point at the center of the sample is smaller than that at the edge. We also find that the value is smaller for the sample with a chip size of 40 µm which prompts fewer total defects. The power density for sudden lifetime drop on samples with inclined sidewall structures is also smaller than those with vertical sidewall structures. Furthermore, we find that the excitation power density corresponding to the highest luminous efficiency is higher than that corresponding to the sudden drop at the beginning of the lifetime. This opens up possibilities for simultaneously achieving high modulation bandwidth and high efficiency between the two inflection points.